1. Field of the Invention
The present invention relates to a gear drive unit suitable for use as an actuator for air-conditioners used mainly for motor vehicles.
2. Description of the Prior Art
Japanese Patent Laid-open Publication No. 61-44243 discloses a gear drive unit so constructed as to drive two output shafts separately by a single motor. The disclosed gear drive unit includes, as shown here in FIGS. 6 and 7 of the accompanying drawings, a motor 100, a set of spur gears 102, 104 receiving an output from the motor 100 and rotatable in opposite directions, two overrunning or freewheeling clutches 106, 108 for receiving a driving force from the corresponding spur gears 102, 104, and two output shafts 120, 122 receiving the driving force transmitted thereto via the corresponding overrunning clutches 106, 108. All of the components 100-122 jointly constitute a twin-shaft selective output mechanism constructed. With this construction, when an input shaft 124 is rotated by the motor 100 in a forward direction, the output of the motor 100 is transmitted via the overrunning clutch 106 to an output shaft 120. On the other hand, when the motor 100 rotates the input shaft 124 in the reverse direction, the output of the motor 100 is transmitted via the overrunning clutch 108 to an output shaft 122.
Each of the overrunning clutches 106, 108, as best shown in FIG. 7, is composed of a circular inner rotary disk 128, 130 mounted on the input shaft 124, 126 and having at least one pawl 132 (two being shown) at a suitable position on its outer peripheral edge portion, and a cup-shaped outer rotary disk 134, 136 mounted on the output shaft 120, 122 and having sawtoothed projections or gear teeth on its inner peripheral surface. The pawl 132, held by spring compression against the gear teeth, permits the output shaft 120, 122 to turn freely only under certain conditions.
Since the twin-shaft selective output mechanism is disposed on a large torque side which is downstream of a reduction gear, not shown, operatively connected between the motor 100 and the input shaft 124, the overrunning clutches 106, 108 which constitute a part of the twin-shaft selective output mechanism must have a mechanical strength large enough to withstand such large torque. In addition, the overrunning clutches 106, 108 are complicated in construction and hence require an increased number of processing steps.